The present invention relates to a magnet roll used for carrying developer in electrophotography, electrostatic recording, and so on.
Most of magnet rolls used as developing rolls or cleaning rolls in conventional electrophotography or electrostatic recording have such a structure as shown in FIG. 1. In FIG. 1, reference numeral 1 represents a permanent magnet member. The permanent magnet member 1 is formed integrally into a columnar shape, for example, out of sintered powder of magnet material such as hard ferrite, or out of a mixture of ferromagnetic material and binder. A shaft 2 is coaxially fixed in the center of the permanent magnet member 1.
A plurality of axially extending magnetic poles (not-shown) are provided in the outer circumferential surface of the permanent magnet member 1. Next, flanges 3 and 4 are mounted on the shaft 2 at its opposite ends rotatably through bearings 5 and 5. A sleeve 6 formed into a hollow cylindrical shape is fixed to the flanges 3 and 4. The flanges 3 and 4 and the sleeve 6 are made of non-magnetic material such as an aluminum alloy, stainless steel, or the like. Reference numeral 7 represents a sealing member which is fixed between the flange 3 and the shaft 2. The permanent magnet member 1 is usually made to have a diameter in a range of from 15 to 60 mm and a length in a range of from 200 to 350 mm.
According to the above configuration, with the relative rotation between the permanent magnet member 1 and the sleeve 6 (for example, the permanent member 1 is fixed, and the flange 4 is rotated), magnetic developer is absorbed onto the outer circumferential surface of the sleeve 6 to thereby form a magnetic brush to make it possible to perform predetermined working of development, or the like.
In such a magnet roll, it has been known that it is effective to make the surface of the sleeve 6 rough in order to improve the performance of carrying magnetic developer from a developer tank to a development area. As a method of making the surface of the sleeve 6 so rough, there is, for example, a method of giving unshaped sand blasting to the surface of the sleeve 6, as disclosed in U.S. Pat. No. 4,597,661. According to this method, as its superior advantage, it is possible to stir developer by the roughened surface of the sleeve 6 to thereby maintain a proper charged condition, and also stabilize the layer thickness of the developer absorbed on the sleeve 6.
Further, in order to form a good condition of the surface of the sleeve 6 after the above-mentioned blasting, there have been proposed a method using shaped or spherical glass beads as blasting particles (for example, refer to Japanese Patent Unexamined Publication No. Sho-57-116372), a method using unshaped blasting particles together with shaped blasting particles (for example, refer to Japanese Patent Publication No. Hei-2-45189), a method using a mixture of spherical particles and unshaped particles (for example, refer to Japanese Patent Unexamined Publication No. Hei-2-204764), and so on.
Such various proposals have been created to improve the defects in the method using unshaped particles of Al.sub.2 O.sub.3, SiO.sub.2, or the like, in which not only the thickness of a developer layer absorbed and held on the toughened sleeve 6 becomes uneven, but the sleeve 6 is apt to be worn out because of the sharpened shapes of its roughened surface, so that the life of the sleeve 6 becomes short. However, the proposals have problems as follows.
First, in the proposal using glass beads (hereinafter referred to as "FGB"), blasting particles are so brittle as to be easily broken by an impact at the time of collision with the surface of the sleeve 6. Accordingly, there is a problem that the time to maintain predetermined particle size becomes so short that the lifetime is short. In this proposal, therefore, there is a disadvantage that it is necessary to often perform classification of used particles and supply of new particles.
In the proposal using steel balls or stainless steel beads, there is indeed an advantage that they are more durable than the above-mentioned FGB, so that the lifetime thereof is made long comparatively, but the density of material constituting them is so large that it is difficult to control the intensity of spray to the sleeve 6. In the proposal, accordingly, there is a problem that the blasting condition is narrow.
Moreover, in the proposal using steel balls, ferrite particles, or the like, as blasting particles, the balls or particles are constituted by magnetic material so that blasting particles are absorbed onto the surface of the sleeve 6 when blasting is performed in the state where the permanent magnet member 1 is incorporated in the sleeve 6 (usual manner). In this proposal, accordingly, there is a problem that the roughness formed in the surface of the sleeve 6 is apt to be uneven.